Most commercial lift trucks have mast supported carriages which are lifted by means of hydraulic jacks, chain drives, or combinations of both systems. One problem often presented by such lift trucks is that of limited operator visibility. The usually required centralized placement of the mast assembly lift cylinder, relative to the supporting mast, often blocks the vision of an operator located in the cab or operator station of the lift truck.
Another problem presented by such lift trucks is that of excessive unextended mast height. Lift trucks are often required to operate in areas where the ceiling is intermittently low. This requires that such vehicles present a relatively low profile when moving from one load space to another. However, by providing a relatively short fixed mast assembly to permit maneuverability in low ceiling areas, the maximum lift height of the mast assembly may be unduly limited.
Some attempts have been made to address the free lift problem while retaining forward visibility. One solution is set out in U.S. Pat. No. 4,030,568, issued June 21, 1977 to Lloyd K. Heinold. A pair of conventional hydraulic jacks are mounted, one slightly outboard of each of the outer mast members of the mast assembly. Sheaves are mounted to the rods of the jacks, with chains being reeved over the sheaves and being attached to the fixed uprights and the carriage. As the rods extend, the carriage proceeds upward in free-lift operation (operation wherein the carriage moves upward but the overall height of the mast assembly is not extended). Free lift is advantageous in that a load may be picked up and maneuvered without increasing the overall height of the vehicle. This is most important in low ceilinged buildings. After the rods have extended the free-lift distance, they engage the movable mast members whereupon the carriage proceeds upwardly with the movable mast members to a height above that of the fixed uprights. However, not as much height is attained as is desirable.
With such mast assemblies as are disclosed in the Heinold patent no cross-bracing is present between the cylinder rods during free lift operation and no connection exists between the rods and the uprights. During free lift, the overall assembly is, thus, not cross-braced. Thus, during free lift, the unbraced jacks are subjected to the weight carried by the carriage as well as to accidental jolts. Because of the lack of cross-bracing, this can lead to misalignment of the jacks with the movable mast whereby the engagement of the rods with the movable mast may be impaired, or in extreme cases even prevented. Also, the jacks are limited in length so as to allow free lift operation. This creates a limit on the overall height to which the carriage can be raised.
It would be desirable to provide a high visibility mast assembly which retained the advantages of the apparatus disclosed in the Heinold patent but which was fully braced during free lift operation and thus was not subject to possible alignment problems, and which would allow lifting the carriage to above the height attainable with the Heinhold apparatus.
The present invention is directed to overcoming one or more of the problems as set forth above.